Hydrocarbon based lubricants oxidize over time when exposed to heat and oxygen (air) that are ubiquitously present during their manufacture, transport, storage, or use. Uncontrolled oil oxidation produces harmful species, which eventually compromise the designated functionalities of the lubricant, shortens its service life and, to a more extreme extent, damages the machinery it lubricates. For the prevention of lubricant oxidation, a practical approach is to use a suitable antioxidant system, comprising of one or more active components.
Driven by escalating performance and environment requirements for many classes of lubricant products, the industry is continuously seeking high performance antioxidants to work with modern lubricant formulations in order to provide better oxidative stability, longer drain interval, improved low temperature properties, and greater fuel economy.
One notable change from the lubricant formulation point of view is the reduction in the use of zinc dialkyldithiophosphates (ZDDP). Over the past decades, ZDDP's have been an important class of lubricant additive for many types of lubricants owing to their superior cost-effectiveness in wear protection and oxidation inhibition, particularly through a synergistic action with primary antioxidants. The presence of zinc in ZDDP's, however, contributes to the formation of ash particulates in internal combustion engines, and the volatile phosphors after entering the exhaust stream poisons the NOx catalysis, both shortening the useful life of catalytic converter.
One low ZDDP antioxidant composition is described in U.S. Pat. No. 3,305,522, the entirety of which is incorporated herein by reference, which describes hindered phenolic compounds prepared by a two-step process which involves reacting one mol of dicyclopentadiene with at least one mol of a phenolic compound selected from the group consisting of phenol, para-cresol, mixed meta-para-cresol and para-ethyl phenol in the presence of a Friedel-Craft type catalyst. This reaction product is subsequently alkylated with at least one-half mol of a tertiary olefinic material per mol of dicyclopentadiene, the tertiary olefinic material being selected from the group consisting of isobutylene, tertiary hexenes and tertiary pentenes. The hindered phenolic compounds described in the '522 patent possess antioxidant properties for stabilizing organic materials such as rubber, gasoline, and oils.
A second low ZDDP antioxidant composition is described in U.S. Pat. No. 6,559,105, the entirety of which is incorporated herein by reference, which discloses a composition of an antioxidant of the formula:
wherein R3 is an alkyl group of 2 to 6 carbon atoms, and a dispersant or a detergent, is a useful additive package for lubricant compositions. Hindered phenols of the type described in the '105 patent, however, recently have been shown to decompose under high temperature oxidation conditions, thereby decreasing their antioxidant properties.
The need remains for additional antioxidant compositions, particularly for lubricant applications, having low amounts of ZDDP and high stabilizing effectiveness. The need also exists for antioxidant compositions that remain stable at high temperature oxidation conditions.